The present invention relates to fluids which gel following a trigger event for subterranean well and formation treatment.
At various stages in the drilling, completion, and operation of natural hydrocarbon reservoirs, gelling compositions are employed. The procedures in which they are used include fracturing subterranean formations, modifying the permeability of subterranean formations and sand control. Other procedures are known as plugging operations. Plugging an oil well is a common operation practised for a variety of reasons, for example, to sidetrack above a fish (equipment lost in the hole), to initiate directional drilling in a weak formation, to plug back a zone or a complete well for abandonment, to cure a lost circulation problem encountered during drilling, or to provide a test anchor when a weak formation exists in an open hole below the zone to be tested.
Many gelling compositions are based on crosslinked polymers. The polymers employed are typically water soluble. Commonly used classes of water soluble polymers include polyvinyl polymers, polyacrylamides, cellulose ethers, or polysaccharides, in particular galactomannans (e.g., guar gum) and derivatives, thereof, or heteropolysaccharides (e.g., xanthan gum).
With respect to crosslinking agents, numerous examples of both, organic and inorganic compounds are known to those skilled in the art. Exemplary inorganic crosslinking agents include typically polyvalent metals, chelated polyvalent metals, and compounds capable of yielding polyvalent metals. Some of the more common inorganic crosslinking agents include chromium salts, iron salts, vanadium salts, aluminates, borates, gallates, titanium chelates, aluminium citrate, chromium citrate, chromium acetate, chromium propionate, and zirconium chelates or complexes.
Exemplary organic crosslinking agents include include aldehydes, dialdehydes, phenols, substituted phenols, and ethers. Phenol, resorcinol, catechol, phloroglucinol, gallic acid, pyrogallol, 4,4xe2x80x2-diphenol, 1,3-dihydroxynaphthalene, 1,4-benzoquinone, hydroquinone, quinhydrone, tannin, phenyl acetate, phenyl benzoate, 1-naphthyl acetate, 2-naphthyl acetate, phenyl chloroacetate, hydroxyphenylalkanols, formaldehyde, paraformaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, isobutyraldehyde, valeraldehyde, heptaldehyde, decanal, glyoxal, glutaraldehyde, terephthaldehyde, hexamethylenetetramine, trioxane, tetraoxane, polyoxymethylene, and divinylether are some or the more typical organic crosslinking agents.
Of specific interest with regard to the present invention are so-called xe2x80x9cflow-initiated gellingxe2x80x9d liquids or emulsions. According to present knowledge, these liquids thicken when subjected to high shear stress and/or a pressure drop across a sudden constriction in the flow path such as the nozzels of a drill bit. The forces cause the emulsion to break down and initiates reactions between molecules from different phases within the emulsion. The fluid must have physical characteristics which enable it to be easily pumped through a drillstring or coiled tubing. The sudden deformation of the fluid induced when the liquid is forced through a sudden constriction at a high pressure drop results in a thickening or gelling of the liquid. It is desirable that the time before thickening is sufficient for the liquid to flow into a predetermined location in the wellbore or into the formation surrounding the wellbore.
When employed as an emulsion, i.e., a composition containing immiscible phases, usually a stabilizing agent or surfactant is added. A surfactant molecule is characterised by containing portions which are strongly attracted to each of the phases present, e.g., in case of a water/oil emulsion, hydrophilic and a hydrophobic portions.
U.S. Pat. No. 4,663,366 discloses an oil-water based liquid of such a nature and an emulsion technique for controlling the thickening of the liquid. According to said patent, a polycarboxylic acid-containing water-in-oil emulsion is used wherein the oil phase contains dispersed therein a hydratable, water-swelling, hydrophilic clay such as bentonite, and additionally contains a solubilized surfactant consisting of a polyamine derivative, and the aqueous phase contains dissolved therein a polyacrylamide and a polycarboxylic acid.
It is critical that the clay be kept separated from the water until the required thickening of the liquid is initiated. For that purpose each droplet of the dispersed aqueous phase is coated with a membrane or a film of a polymeric material which is formed as a consequence or the aqueous phase being dispersed or emulsified in the oil phase of the emulsion. The film or membrane is formed as a result of the interfacial interaction between the polyamine derivative in the oil phase and the polyacrylamide and the polycarboxylic acid in the dispersed aqueous phase.
When the emulsion is subjected to high shear forces, the protective film around the dispersed droplets in the emulsion is broken and the bentonite comes into contact with the water, thus enabling a swelling of the bentonite and consequently a thickening of the liquid.
A similar dual internal phase composition comprising water and clay particles in separated droplets is described in the U.S. Pat. No. 4,397,354.
Another downhole composition with two internal phases is described in the U.S. Pat. No. 4,819,723. The external oil phase includes an aqueous solution of alkali metal silicate and a polyvalent cation in a second phase to provide additional salinity. The emulsion is broken by microwave radiation. Another alkali metal silicate containing microemulsion is described in the U.S. Pat. No. 4,799,549.
Yet another dual internal phase well treatment fluid is described in the British Patent application GB-2022653. Using for example a water-in-oil emulsion dispersed in an aqueous medium, viscosifiers or acids are added to the external phase to provide separation between two ingredients.
Other partly dual internal phase compositions are found in the U.S. Pat. No. 4,891,072, describing the use of such systems as multi-component grouting composition.
A single internal phase composition is described in the British Patent application GB-2008651. The internal phase contains polyacrylamide (PAA), injected into the formation in a process for shutting of water.
In the International Patent Application WO 94/28085, an alternative fluid is disclosed, consisting of an emulsion of a continuous oil phase containing an emulsifier and a crosslinking agent for a polysaccharide and a water discontinuous phase containing a polysaccharide. When this liquid is subjected to sudden deformation the emulsion is broken causing the polysaccharide to crosslink and form a gel which is much stronger than that of U.S. Pat. No. 4,663,366. The particularly preferred system consisted of xanthan as the polymer and calcium hydroxide as the crosslinker. The gel formed by this system remains intact at temperatures below 90xc2x0 C., above this temperature the gel breaks down.
Though having many advantageous properties, the known emulsion systems are restricted in use. The variety of crosslinking agents which can be used is restricted by the requirement for them to be compatible with the oil chase making it difficult to select a polymer-crosslinker system optimized for a specific application.
It is therefore an object of this invention to provide improved emulsion systems suitable for a broader range of oilfield applications. It is a specific object of the invention to broaden the range of applicable polymers and crosslinking agents for such systems.
The objects of the invention are achieved by a composition and method as set forth in the appended independent claims.
An important feature of the present invention is pumping an emulsion comprising at least two discontinuous phases containing a crosslinking agent and a polymer in similar or identical solvents while maintaining two phase boundaries to separate both components until the phase boundaries are ruptured though a flow-initiated force. The flow-initiated force is preferably generated by pumping the emulsion through one or more flow restrictions, preferably at a pressure drop in the range of 10 to 50 bar.
A Preferably, the crosslinking agent and the polymer are dissolved in an aqueous solution separated by an hydrophobic phase. The hydrophobic phase can be either continuous or discontinuous. It may advantageously be selected from mineral oils, vegetable oils, esters and ethers. The hydrophobic phase should also contain a suitable surfactant and may be viscosified with and organophillic clay or other additives to enhance the desirable properties of the emulsion produced.
A preferred emulsifier comprises a siloxane or silane based composition as described for example in the UK Patent Application GB-A-2113236 or the International Patent Application WO 96/07710.
In addition to the organic polymers already discussed it is possible to form desirable gels from inorganic compounds such as silicates.
The novel emulsions in accordance with the invention are preferably used for remedial operations in a wellbore or in subterranean formations. However, they may have advantageous use outside the oilfield industry, for example for the building industry.
These and other features of the invention, preferred embodiments and variants thereof, and further advantages of the invention will become appreciated and understood by those skilled in the art from the detailed description and drawings following hereinbelow.